As for the pressure plate, it is used to "seal" the "box" he has made out of the floor of the trunk. It is basically like any other sealed sub box, but he decided to make one side glass.

But in the first pic http://www.kalliojarvi.net/kg/golf/golf-10-e.htm there is 23 litres sealed box under the glass in spare tire hole and yes the glass protects but as pressure plate glass isn't sealed, sound pressure flows from sides inside car.

That glass can be taken away and play music just like with the glass but what it does??? Is it used as some kind of Transmission Line?

Pressure plate

Hello all.

System with pressure plate is like closed bandpass box with really small tuned chamber, really short port and very high tuning frequency.

Normal closed box works as closed rear chamber of the bandpass box. "Pressure plate" is about 60mm away from the box and the space between box & plate works as tuned chamber of the bandpass box. In the pressure plate there is a hole which size is about 25% of the cone area of used subwoofer. The hole works as the port of the bandpass box. Length of the port is same as the thickness of the pressure plate. So the tuning frequency is so high ( much over 100Hz ) that it doesn't mather anything when you are using low lowpass frequency for the subwoofer.

Benefits of the pressure plate is more controlled and accurate bass response.